Obsidium is the low level ore mined in Cataclysm zones Mount Hyjal and Deepholm. It comes in both regular Obsidium deposits and Rich Obsidium deposits.
You can also mine the pet Elementium Geode. This pet does sell well, and you can make a bit extra gold by leveling it to 25 first.
You can also farm the following gems and elementals:
- Volatile Earth
- Volatile Air
- Inferno Ruby
- Ocean Sapphire
- Dream Emerald
- Ember Topaz